The first time we saw one was completely random at the Jenolan Caves river outside! Didn't expect to see one. And the second time was at the same spot as you @traveltramp - my partner actually did a river dive (not in the broken river) and it said that you can potentially dive with platypus it's pretty rare though. We really loved the area, we stayed at cottage at the top of the mountain, went to see the platypus in the morning and we even did some flying fox in the forest which was nice too.